Advertisement

Advertisement

Forum : A weather eye on the sunspots

By E. Norman Lawrence

DO sunspots affect our weather? Astronomers and meteorologists have long
suspected that they do.

Some of the clearest evidence of a link between sunspots and the weather
recently came to light with the discovery of records compiled by the Scottish
medic Thomas Short in the 18th century. He settled in England and collected
medical and social data. But of special interest are his weather data, spanning
1468 to 1766. Only assessments of sunspots are available for dates before 1610
when direct measurements became available.

Before analysing the data, we need to understand how sunspot cycles differ
from each other. In each cycle, the minimum monthly average number of sunspots
is near zero. But the maximum can be anything from about 30 to 200 or more.
There are three keys to relating sunspots to Earth’s weather patterns. The first
is what stage of the sunspot cycle we are in. The second is the size of one or
both adjacent amplitudes. The third is whether we are in a series of cycles of
large or small amplitude.

The pattern that emerges from an analysis of climatological data generally
shows, in each 11-year cycle, two potential rain peaks in the westerlies
temperate zone, which includes Britain and adjacent latitudes. A westerly-type
rain peak tends to occur near or about two years before sunspot maxima,
especially in small-amplitude cycles. A mainly convective or thundery rain peak
associated with weak westerlies tends to occur near or about two years before a
sunspot minimum, notably in a cycle of large amplitude (see
prediction diagram for westerlies zone, below.

Advertisement

The summer drought of 1995 preceded the sunspot minimum of early 1996 and
came after two large sunspot amplitudes. It was caused by very weak westerlies
and an atmosphere too stable to produce convective rain. In early August, the
persistence of the severe hot drought set me searching through Short’s weather
records for similar conditions—I found six. Five were at or close to a
sunspot minimum. Of these five hot summer droughts, three ended in August and
two in September. Based on this, I forecast that the 1995 drought would end on
31 August or 1 September. It ended on 2 September—in Southeast England, at
least.

Even if the consecutive large sunspot maxima of 1979 and 1989 are followed by
a third large sunspot amplitude (around 2000), Britain’s rain will tend to peak
from 1998 to 2000, on the approach of the next sunspot maximum, probably around
the year 2000.

The diagram shows another result of weak westerlies around or just before
minimum sunspot numbers, especially during large amplitude cycles—a cold
winter anticyclone over land, leading to more fog and frost than usual. Two
disastrous London winter smogs, in 1952 and 1962, were close to minimum sunspot
levels, and there were above-average sunspot amplitudes in 1947, 1957 and
1969.

Correspondingly, persistently weak westerlies beyond the summer of 1995 led
to more fog and frost than average and below-average rainfall in Britain during
the six months to March 1996. Fortunately, the Clean Air Act of 1956 prevented
last winter from suffering the terrible smogs of the past. A similar position in
the sunspot cycle and similar weather pattern can be expected around 2005 to
2007.

Interestingly, even a small convection rain peak near the sunspot minimum can
become the main rain peak of the cycle and so reverse the usual relationship
between sunspots and rain. Sceptics use such a rare occurrence as evidence
against the existence of any relationship between sunspot number and weather.
Anyone who doubts such basic correlations should consider that the four extreme
winters that Short noted (early 1614, 1683 to 1684, 1694 to 1695 and 1739 to
1740) all occurred close to sunspot maxima and generally between small sunspot
amplitudes.

The probability of this happening by chance is practically zero and I suggest
that all four extreme winters have a common mechanism. The pronounced northern
or polar high-pressure region, accompanying maximum sunspot levels in
small-amplitude cycles, has moved or expanded seasonally southwards, displacing
milder westerlies. Similar sunspot conditions foster a corresponding mechanism
for hot summer droughts, with the northward movement of a pronounced subtropical
high-pressure belt displacing cooler westerlies.

Long-range forecasts based on solar activity may at last be becoming
practicable.

Advertisement

E. Norman Lawrence is a former Meteorological Office scientist
specialising in weather-sunspot relationships.